The invention relates to an optical arrangement for EUV lithography, in particular a projection lens, and to a method for configuring such an optical arrangement.
Materials which are used as substrates for reflective optical elements in EUV lithography on account of the extremely stringent requirements made of geometrical tolerances and stability which exist particularly in projection lenses of EUV lithography apparatuses, are permitted to have only a very low coefficient of thermal expansion (CTE) in the temperature range used there. The substrate materials used there typically comprise two constituents, the coefficients of thermal expansion of which have a mutually opposite dependence on temperature, such that the coefficients of thermal expansion mutually compensate for one another virtually completely in the range of the operating temperature.
A first group of materials that meets the stringent requirements of the CTE for EUV applications comprises doped silicate glasses, e.g. silicate or quartz glass doped with titanium dioxide, which typically has a silicate glass proportion of more than 80%. One such silicate glass that is commercially available is sold by Corning Inc. under the trade name ULE® (Ultra Low Expansion glass). TiO2-doped quartz glass can, if appropriate, also be doped with further materials, e.g. with materials that reduce the viscosity of the glass, as is presented e.g. in US 2008/0004169 A1, wherein alkali metals are used, inter alia, in order to reduce the effects of striae in the glass material.
A second group of materials suitable as substrates for EUV mirrors comprises glass ceramics, in which the ratio of the crystal phase to the glass phase is set such that the coefficients of thermal expansion of the different phases virtually cancel one another out. Such glass ceramics are offered e.g. under the trade name Zerodur® from Schott AG or under the trade name Clearceram® from Ohara Inc.
The dependence of the thermal expansion (change in length) of the above-described materials on temperature is parabolic in the relevant temperature range, that is to say that there exists an extremum of the thermal expansion at a specific temperature. The derivative of the thermal expansion of zero expansion materials with respect to temperature, i.e. the coefficient of thermal expansion, is approximately linearly dependent on temperature in this range and changes sign at the temperature at which the thermal expansion is extremal, for which reason this temperature is designated as zero crossing temperature. Consequently, the thermal expansion is minimal only for the case where the operating or working temperature of the substrate coincides with the zero crossing temperature. In the case of small deviations from the zero crossing temperature, although the coefficient of thermal expansion is still low, it increases further with increasing temperature difference with respect to the zero crossing temperature.
It is known that the zero crossing temperature of TiO2-doped quartz glass can be altered by the setting of the titanium dioxide proportion during the production of the quartz glass (at temperatures above the softening point). In the case of conventional TiO2-doped quartz glass, the gradient of the coefficient of thermal expansion in the vicinity of the zero crossing temperature typically lies in the range of between approximately 1 ppb/K2 and 3 ppb/K2.
It is known from US 2011/0048075 A1 that the zero expansion temperature of TiO2-doped quartz glass (without exceeding the softening point) can be set to a specific value within a predefined range of values by means of a final heat treatment step. In this way, a plurality of substrates having different zero expansion temperatures can be produced from partial volumes of one and the same boule (quartz glass block having an (approximately) constant TiO2 proportion). The temperature range within which the zero expansion temperature is intended to be able to be set by means of the additional heat treatment step is approximately +/−10° C. or approximately +/−5° C. The gradient of the coefficient of thermal expansion in the vicinity of the zero expansion temperature is intended not to vary or to vary only slightly in this case.
It is likewise known that through a suitable choice of the temperature/time curve during a heat treatment process for the glass, the fictive temperature of the glass and thus not only the zero crossing temperature but also the gradient of the coefficient of thermal expansion at the zero crossing temperature can be set. Thus, by way of example DE 21 40 931 C3 has disclosed a method for setting the coefficient of thermal expansion of a TiO2—SiO2 glass consisting of 12% (by weight) to 20% (by weight) TiO2, wherein the sign of the coefficient of thermal expansion can be changed from negative to positive by subjecting the glass to a thermal treatment at a temperature of from 700 to approximately 900° C.
Given a high TiO2 content of the quartz glass of e.g. more than 10%, the problem can occur that there is a tendency for Ti-rich particles to crystallize out, which can adversely influence the polishability of the quartz glass. US 2011/0052869 A1 discloses applying, to a quartz glass base body having high TiO2-doping (10% or more) a layer composed of a more weakly doped quartz glass, which can be polished to a higher surface quality than the quartz glass base body.
EP 1 608 598 B1 has disclosed a TiO2-containing quartz glass which is intended to have a fictive temperature of at most 1200° C., a concentration of OH groups of at most 600 ppm and a coefficient of thermal expansion of 0+/−200 ppb/K between 0° C. and 100° C. During production, the quartz glass is held for a predetermined time period at a temperature which exceeds 500° C., and the temperature is subsequently decreased at an average cooling rate of at most 10° C./h to 500° C.
US 2007/0035814 describes a projection lens for wavelengths of less than 157 nm, which comprises at least two optical elements composed of different materials, which differ in the region of the zero expansion temperature in terms of the gradient, more particular in terms of the sign of the gradient of the coefficient of thermal expansion. Examples of the different materials specified include firstly glass ceramics, in particular Zerodur®, and secondly amorphous titanium silicate glass, in particular ULE®.
WO 2004/015477 A1 discloses setting the zero crossing temperature in the material of an optical component such that this substantially (except for approximately +/−3 K) corresponds to a maximum temperature to which the optical component is heated. Image aberrations of an optical system are intended to be minimized in this way. US 2003/0125184 describes a method for producing a glass ceramic having a zero crossing temperature that deviates from a desired temperature by at most +/−10° C.